How to have a boundary box around a whole group instance?
I wrote a small addon for another question that creates a bounding box around all selected objects. It seems it needed a little update to work in current versions of blender which is now available.

This does not create a dynamic bounding box, it will only match the objects you have selected at the time you create it, this can just be deleted and re-created whenever you needs updating.

After selecting one object you can use &#8679; ShiftG->Group to select all objects of the same group before creating the bounding box.